Try adding darker values around the castle to give it a Better feeling of depth. It blends too much with the mountain and can be a bit difficult to notice. Also add highlights to the castle a bit more.

Geez, this is pretty damn great! I love the greyscale value and detail!!! Shame how my pretentious art has gotten on the popular list twice while your amazing work gets no attention :/ you should put this on deviantart, you have a lot of potential .